1. Field of the Invention
The invention pertains to data communications systems, and particularly to diversity handover (e.g., soft handover) in a telecommunications system such as a wideband code division multiple access telecommunications system.
2. Related Art and other Considerations
In a typical cellular radio system, mobile stations (MS), also known as mobile user equipment units (UEs), communicate via a radio access network (RAN) to one or more core networks. The mobile stations (MSs)/user equipment units (UEs) can be mobile telephones (“cellular” telephones) and laptops with mobile termination, and thus can be, for example, portable, pocket, hand-held, computer-included, or car-mounted mobile devices which communicate voice and/or data with radio access network.
The radio access network (RAN) covers a geographical area which is divided into cell areas, with each cell area being served by a base station (also known in some networks as a “B-node” or “node-B”). A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by a unique identity, which is broadcast in the cell. The base stations communicate over the air interface (e.g., radio frequencies) with the mobile stations within range of the base stations. In the radio access network, several base stations are typically connected (e.g., by landlines or microwave) to a radio network controller (RNC). The radio network controller, also sometimes termed a base station controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UTRAN is a third generation system which in some respects builds upon the radio access technology known as Global System for Mobile communications (GSM) developed in Europe. UTRAN is essentially a wideband code division multiple access (W-CDMA) system. An undertaking known as the Third Generation Partnership Project (3GPPP) has endeavored to evolve further UTRAN and GSM-based radio access network technologies.
As those skilled in the art appreciate, in W-CDMA technology a common frequency band allows simultaneous communication between a mobile station (MS) and plural base stations. Signals occupying the common frequency band are discriminated at the receiving station through spread spectrum CDMA waveform properties based on the use of a high speed, pseudo-noise (PN) code. These high speed PN codes are used to modulate signals transmitted from the base stations and the mobile stations (MSs). Transmitter stations using different PN codes (or a PN code offset in time) produce signals that can be separately demodulated at a receiving station. The high speed PN modulation also allows the receiving station to advantageously generate a received signal from a single transmitting station by combining several distinct propagation paths of the transmitted signal. In CDMA, therefore, a mobile station (MS) need not switch frequency when handoff of a connection is made from one cell to another. As a result, a destination cell can support an additional leg of a connection to a mobile station (MS) at the same time the origination cell continues to service the original leg of the connection. Since the mobile station (MS) is always communicating through at least one cell during handover, there is no disruption to the call. Hence, the term “soft handover.” In contrast to hard handover, soft handover is a “make-before-break” switching operation.
Suppose that a mobile station already has a leg of a connection established with a base station (the source base station) serving a cell (the source cell) in which the mobile station presently resides. Either periodically or as triggered by certain events, the mobile station measures and reports to a control node (e.g., a radio network controller [RNC]) the received signal strength of preselected transmissions (e.g., pilot signals) from various base stations. In the W-CDMA context, a measurement report sent from the mobile station to the control node includes signal strength measurements for cells (e.g., base stations) already in an “active set” (cells for which diversity handover is already applicable), as well as other monitored cells. As the mobile station travels toward a destination cell (served by a destination base station) which is not in the active set, the mobile station eventually hears the pilot signal from the destination base station, and includes the destination base station in its measurement report to the control node. Eventually a decision must be made by the radio access network whether to add a new leg of the connection with the mobile station (the new leg involving the destination base station) by initiating a soft handover sequence at the destination base station.
Traditionally the radio access network determines to initiate a handover sequence at the destination base station in accordance with a soft handover algorithm. In the W-CDMA context, the soft handover algorithm has various events here of interest. A first event (Event 1A) is Radio Link Addition, which occurs when the measured and filtered pilot signal from the destination base station (not in the active set) exceeds a certain handover threshold. That certain handover threshold, herein also known as a Fixed Offset Threshold, is a fixed offset from the best (greatest strength) pilot signal in the active set (e.g., the source cell), as set forth in Expression 1. The fixed offset is a properly chosen constant. A low fixed offset means that a fixed offset threshold is high, and consequently a high signal strength is required to start handover. For a high fixed offset, the converse occurs. A description of how the fixed offset is chosen in provided in Third Generation Partnership Project Technical Specification 25.331.FixedOffsetThreshold=SignalQuality(Best Cell)−FixedOffset  Expression 1A second event (Event 1B) is Radio Link Removal, which occurs when the measured and filtered pilot signal from the destination base station falls below the threshold of Expression 1. For the events (such as Event 1A and Event 1B) to occur, typically the pilot signal must maintain its strength for a predetermined trigger time and a certain hysteresis value may be factored into the threshold expression. Moreover, for the Radio Link Addition event, the active set must not be full. A more detailed understanding of the W-CDMA soft handover algorithm, including other events and conditions, can be ascertained from Radio Resource Management Strategies, 3G TR 25.922, Ver. 0.5.0, September 1999, generated by the Third Generation Partnership Project, Technical Specification Group RAN, Working Group 2 (WG2).
The point where the handover sequence is started can be said to define the cell border between two cells. If the cell border is situated too far from the source base station, the mobile station might lose contact with the source base station before the handover is completed. In such a situation, the call is unfortunately dropped. Furthermore, to keep the signal quality at a reasonable level at the mobile station, the source base station and the mobile station have to increase the transmission power as the mobile station moves away from the source base station. Consequently, the interference in the current cell and to other neighboring cells increases, leading to lower system capacity. On the other hand, if the cell border is close to the source base station, but too far from the destination base station, the destination base station (instead of the source base station) has to start its transmission with a high output power.
With the conventional fixed offset threshold value of Expression 1, the point where a handover is started is essentially the same between the current cell and any of the cells not currently in the active set. Thus, there is no possibility of controlling when to start a handover between two specific cells, and many of the handovers may be started at non-optimal points.
The conventional handover sequence performed by the destination base station involves various activities, usually commencing with allocating resources, and then subsequently, e.g., activating a receiver at the destination base station to be associated with the mobile station, followed by L1 synchronization with the mobile station. However, some of these handover sequence activities, such as L1 synchronization, are considerably complex, process intensive, and time consuming. Such characteristics of these handover sequence activities militate against the overall network goal of efficiency, e.g., of avoiding delay (whether it be call setup delay, delay at channel switching, delay at handover, etc.). In fact, one common denominator and a major reason for delay in many traffic situations is the time required to perform L1 synchronization in different scenarios, including diversity handover.
Various prior art systems (such as the Ericsson CMS88 and CMS30 TDMA systems) have employed verification receivers which enable a target cell to verify the existence of a mobile station, by synchronizing the verification receiver to the mobile station. In essence, output from the verification receiver advises whether the mobile station can be detected in the receiving cell or not. In such systems, a positive verification result is a condition for proceeding with the handover sequence.
U.S. Pat. No. 6,052,598 uses a series of received signal strength measurements of a mobile unit to extrapolate a time at which that mobile unit would have a handover in accordance with fixed offset thresholds, and affords an opportunity to allocate wireless resources for the mobile unit in anticipation of the extrapolated handover.
U.S. Pat. No. 5,530,912 provides, within a cell, a handover region and a pre-handover zone relative to handover to an adjacent cell. When a mobile station is in the pre-handover zone, a free channel is reserved in the adjacent cell. The free channel in the adjacent cell is not granted until the mobile station moves into the handover region.
What is needed, therefore, and an object of the present invention, is a technique for expediting time-intensive handover activities and thereby reducing diversity handover delay.